The present invention relates to the field of semiconductor device fabrication, and more particularly to improved methods for etching high aspect ratio contact openings in oxide layers.
In the fabrication of semiconductor devices, numerous conductive device regions and layers are formed in or on a semiconductor substrate. The conductive regions and layers of the device are isolated from one another by a dielectric or insulating layer, for example, silicon dioxide or a doped oxide such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG). These dielectric layers typically overlay a silicon-comprising surface such as single crystal silicon, epitaxial silicon, polysilicon, or suicides such as titanium silicide.
At several stages during wafer fabrication, it is necessary to form contact openings through insulative material to establish electrical communication with the integrated circuitry. Such contact openings, when filled with a conducting material such as a metal or polysilicon, electrically connect devices with the integrated circuitry. To ensure formation of desired dimensions and profile for contact openings, the etchant must be highly selective to promote removal of the insulation layer and not the underlying layer.
Contact openings with high aspect ratios, i.e., a high height-to-width ratio, are formed to be fairly narrow, typically with vertical sidewalls to ensure that a sufficiently large contact area is provided at the bottom of the contact opening for conductive material that is subsequently formed in the opening. To form the openings, a masking layer such as a photoresist is formed over the insulative layer, i.e., silicon oxide layer, and is subsequently patterned to define the contact openings. The contact opening is etched using an etch that is highly selective relative to masking layer. Conventional processes used to form a contact opening involve etching through the insulative layer by exposure to a plasma formed in a plasma reactor. Reactive ion etching (RIE) and plasma etching (PE) are common dry-etch plasma methods used to open contact openings anisotropically through a dielectric.
A fluorocarbon plasma is typically used to etch silicon dioxide. Such a plasma typically includes one or more fluorocarbons as the primary active constituents, for example CF4, CHF3, and C3F8, CH2F2, CH3F, C2F6, C4F6, CnFn+4, and mixtures thereof. The fluorinated gas dissociates and reacts with the silicon oxide to form volatile silicon difluoride (SiF2) or silicon tetrafluoride (SiF4) and carbon monoxide or carbon dioxide.
Although the plasma etch rate of the oxide is generally faster than the resist erosion rate, when dry etching an opening having a high aspect ratio, the etch chemistry causes the resist layer to gradually erode away, often before the desired depth of the opening is achieved. Another problem is related to faceting or chamfering of the photoresist mask at the edge of the opening, caused by ion bombardment. This can wear away the underlying oxide resulting in surface roughness and striations in the etch features, and the loss of critical dimensions of the opening being etched. In an array such as a memory cell, contacts are positioned in close proximity to each other, and the erosion and localized breakdown of the photoresist can result in the development of notches and other blemishes in the surface of the contact, which can extend to and short an adjacent contact.
The plasma etching processes generates very reactive ionized species, atomic fluorine, and CxFy radicals that combine to form polymeric residues. A drawback with plasma etching and RIE of silicon oxide using some fluorinated etch gases is the buildup of carbon-fluorine based polymer material on the sidewalls of vias and other openings that can deposit during the etch. FIG. 1 illustrates a wafer fragment 10, a contact opening 12 etched through an opening in a mask 14 downwardly through a silicon oxide layer 16 deposited on a substrate 18, and the effect of the buildup of polymeric residues 20 on the sidewalls 22 and the bottom surface 24 of the contact opening 12 formed during a typical prior art etch process.
The continuous buildup of polymeric etch residues on sidewalls 22 of the oxide opening 12 tends to constrict the opening, inhibiting the etch and resulting in the profile of the sidewalls becoming tapered, as depicted in FIG. 1. Polymer material can also build up in the bottom 24 of the opening 12. Since an anistropic etch of the oxide layer 16 relies on ionic bombardment of the bottom 24 of the opening being etched, if too much polymeric layer 20 deposits on the bottom surface 24 of the opening during etching, etching of the contact opening will cease if the layer 24 is not removed.
Therefore, a need exists for a method of etching silicon oxide layers to provide high aspect ratio openings that overcomes these problems. It would be desirable to provide an etching process for the formation of deep contact openings through an oxide layer that inhibits or regulates the deposition of polymeric etch residues on the sidewalls of the openings and improves resist selectivity and eliminates striations and notching.
The invention provides an improved process for plasma etching of a silicon oxide layer to form a via or other contact opening while controlling the deposition of polymeric residues on the surface of a mask layer and the sidewalls and bottom surface of the contact opening. In particular, the invention improves resist selectivity and reduces striations by the addition of nitrogen-comprising gases such as NH3 to fluorocarbon (CxFy) and fluorohydrocarbon (CxFyHz) etch chemistries.
The etching is performed by exposing the silicon oxide layer through a mask opening to an etch gas in an ionized state in a reaction chamber of a plasma-generating device. The etch gas includes one or more organic fluorine-comprising gases such as CF4, CHF3, CH2F2, among others, and can include one or more nitrogen-comprising gases. Suitable nitrogen-comprising gases are those that do not substantially etch the resist layer and/or deposit or build-up polymer on the mask layer. Exemplary nitrogen-comprising gases include N2O, NH3, N2H4, and RNH2 where R is a C1-C3 hydrocarbon or fluorohydrocarbon, among others. The etch gas can optionally include one or more inert carrier gases such as argon or helium. The etching is preferably performed by reactive ion etching or plasma etching. The etch gas can be exposed to a microwave electric field and/or a magnetic field during the etching step.
In one embodiment of the invention, the method involves etching the layer of silicon oxide to provide an opening extending therethrough by exposing the silicon oxide layer through a mask opening to a first etch gas and then a second etch gas, in an ionized state in a reaction chamber of a plasma generating device. A first etching is performed by exposing the silicon oxide layer to etch a contact opening through the silicon oxide layer, preferably to a depth of at least about 0.5 micron, and an aspect ratio of at least about 2:1. The first etch gas includes at least one organic fluorocarbon and, optionally, one or more nitrogen-comprising gases in a minor amount such that there is essentially little or no polymeric material formed on the mask layer and the silicon oxide layers during the etching step.
A second etching is then performed by exposing the silicon oxide layer to a second etch gas to increase the opening downwardly through the silicon oxide layer while a polymeric material is formed on the mask layer during the etching step. The second etch gas includes at least one organic fluorocarbon and an effective amount of at least one nitrogen-comprising gas to reduce the resist etch rate and/or provide formation of polymeric material on the mask layer during the etching step. The second etch gas can be provided as a separate gas or by increasing the amount of the nitrogen-comprising gas of the first etch gas to reduce the etch rate of the resist and/or deposit polymeric material on the mask during the etching step to inhibit erosion of the mask layer. Upon depositing a layer of the polymeric material that protects the photomask layer opening, the silicon oxide layer can be further etched to increase the depth of the opening by exposing the oxide layer to the first etch gas, or by decreasing the amount of the nitrogen-comprising gas in the second etch gas to a level whereby formation of polymeric material on the mask is substantially suppressed during the etching step.
In another embodiment of the invention, the method involves etching a contact opening through a silicon oxide layer by exposing the silicon oxide layer to a first etch gas to partially etch the opening while a polymeric material is formed on the surface of the mask, and then to a second etch gas to further extend the opening downward while suppressing deposition of the polymeric material. The first etch gas includes at least one organic fluorocarbon and an effective amount of at least one nitrogen-comprising gas to reduce the etch rate of the resist and/or provide formation of polymeric material on the mask layer during the etching step to help maintain the thickness of the mask layer on the silicon oxide layer.
Upon depositing a layer of the polymeric material that protects the photoresist mask or inhibits etching of the photoresist mask, a second etching is performed to extend the opening downwardly by decreasing the amount of the nitrogen-comprising gas in the etch gas or applying a second etch gas that includes at least one organic fluorocarbon and, optionally, a minor amount of one or more nitrogen-comprising gases, whereby formation of polymeric material on the mask is substantially suppressed during the etching step. Upon reducing the layer of polymeric material whereby the contact opening is unconstricted, the silicon oxide layer can be further etched to increase the depth of the opening by exposing the oxide layer to the first etch gas, or by increasing the amount of the nitrogen-comprising gas to reduce the etch rate of the resist and/or deposit polymeric material on the mask during the etching step to inhibit erosion of the mask layer.
It was found that the addition of a nitrogen-comprising gas such as NH3 to the fluorocarbon etch gas helps maintain the smoothness of the surface of the resist layer to eliminate striations in the etch features, helps eliminate faceting and erosion of the mask and increases the resist selectivity, i.e., decreases the photoresist etch rate with respect to the oxide etch rate. Advantageously, the present invention provides a method for achieving an anistropic etch of an oxide layer to provide a contact opening having a high aspect ratio of at least about 5:1 by using and controlling the amount of polymeric material that forms on the mask layer from the etch gases to maintain the integrity of the mask throughout the etch process to help control the resolution or dimension of the diameter of the contact opening.